A phase-locked loop is a circuit including a voltage controlled oscillator (VCO) which is designed to control the VCO to generate an output signal having a predetermined frequency and/or phase relationship with a reference signal. A typical phase-locked loop is shown in FIG. 1.
The phase-locked loop comprises an oscillator 101. The output of the oscillator is output from the phase-locked loop circuit on line 105. Additionally, the output of the oscillator 101 is fed to an input of a phase/frequency detector (PFD) 102. The PFD 102 outputs a signal that is representative of the phase and/or frequency difference between a reference signal on line 106 and the signal output from the oscillator. The PFD output signal is filtered at a low pass filter 103 (loop filter) and fed back into the oscillator 101 as a control signal on line 107. The control signal 107 modifies the frequency of the oscillator. The frequency of the signal output by the phase-locked loop on line 105 can be changed by varying the frequency of the reference signal. Often, the reference signal is generated by a very stable oscillator whose frequency cannot be varied. Therefore, it can be beneficial to include a divider 104 in the loop so that the output frequency of the phase-locked loop can be varied without having to change the frequency of the reference signal.
An oscillator may be used to output an oscillating signal to drive a frequency mixer used in a transmitter or a transceiver. The frequency mixer may be used, for example, to upconvert a signal in a transmit chain and/or to downconvert a signal in a receive chain. A problem with oscillators in such environments is that they may pick up the signal transmitted from the transmitter/transceiver. The transmitted signal perturbs the output frequency of the oscillator thereby contributing significantly to the phase noise suffered by the oscillator. (Phase noise is rapid, random fluctuations in the phase of a wave caused by instabilities in the timing of the zero crossings of the wave, known as jitter. The zero crossings are the points at which a waveform plotted on a graph of output voltage (on the y-axis) against time (on the x-axis) crosses the x-axis.) When an oscillator comprising an inductor suffers interference from the electromagnetic field of a nearby transmitted signal, the transmitted signal can induce a current in the inductor. This can perturb the signal output from the oscillator so as to include frequency components in the interfering frequency band of the transmitted signal in addition to frequency components at the natural frequency of the oscillator.
Phase-locked loops, such as the one shown in FIG. 1, are often used to control the output of such oscillators. The signal output from the PFD 102 is representative of the phase and/or frequency difference between the reference signal on line 106 and the perturbed signal output from the oscillator. The signal output from the PFD 102 also includes high frequency components representative of the sum of the phases and/or frequencies of the reference signal and the perturbed signal output from the oscillator.
The loop filter 103 of the phase-locked loop attenuates frequencies outside of its passband. The passband of the loop filter is chosen so as to attenuate the high frequency components output from the PFD 102 that are representative of the sum of the phases and/or frequencies of the reference signal and the perturbed signal. The transmitted signal will typically have frequency components lying outside the passband of the loop filter 103 in addition to frequency components lying within the passband of the loop filter 103. Consequently, the perturbed signal will have corresponding frequency components. Therefore, in addition to the high frequency components mentioned above, the loop filter will additionally attenuate frequency components output from the PFD representative of the phase and/or frequency difference between the reference signal and the perturbed signal components lying outside the passband of the loop filter. Consequently, the output of the loop filter 103 is a signal representative of the phase and/or frequency difference between the reference signal and the perturbed signal components lying within the passband of the loop filter 103. This output signal is fed as a control signal to the oscillator 101 to modify the frequency of the oscillator.
In summary, the phase-locked loop tracks the disturbances caused by the part of the transmitted signal that lies within the passband of the loop filter. Under the control of the control signal 107, the oscillator modifies its oscillation frequency to reduce these disturbances.
A problem with the phase-locked loop of FIG. 1 is that it is not able to compensate for the disturbances caused by the part of the transmitted signal that lies outside the passband of the loop filter. This is because the part of the signal output from the PFD 102 that would provide the desired compensation signal for these disturbances is attenuated by the loop filter. The control input to the oscillator consequently does not comprise this desired compensation signal. As a result, the frequency of the signal output from the circuit on line 105 does not match the frequency of the reference signal since it is still perturbed as a result of the high frequency components of the transmitted signal. The perturbations are visible as significant sidebands in the signal output from the circuit on line 105.
It will be understood that this problem may apply to a phase-locked loop in any environment in which it is subject to interference from an interfering signal that has frequency components that lie outside the passband of the loop filter.
There is thus a need for an improved phase-locked loop design that reduces the perturbations in a signal output from a controlled oscillator caused by an interfering signal that lies outside the passband of the loop filter.